Sourdough Boule

Preparation

1. Combine flours in a large bowl and add 1 1/2 cups lukewarm water. Stir well until there are no pockets of dry flour. The dough will initially feel stiff and dry.  Cover the bowl and set aside for 30 to 45 minutes. Meanwhile, stir together the salt and 1 tablespoon warm water in a small dish. 
2. Uncover the bowl and add the starter and the salt water mixture. Using your hands, work the starter into the flour mixture by folding, pinching and twisting the dough. The two textures will be hard to combine initially and may feel soupy but keep working until it starts to bind into one sticky dough. Once combined, begin to work the dough in the bowl. Slide your hand to the bottom of the dough and pull the dough up and over the top. Give the bowl a quarter turn and repeat. Continue pulling and stretching the dough until the dough feels soft and is a little tacky but releases from the side of the bowl when stretched, about 10 minutes. Check the dough by taking a small bit and gently stretching it between your fingers. If it can stretch thin enough to see light through without tearing the gluten has been developed enough and it’s ready. If the dough easily tears, continue to work it for a few more minutes and check again. Scrape any dough bits from the sides of the bowl, cover and set aside for 45 to 60 minutes.
3. Uncover the dough. Using a wet hand, slide your hand down the side of the bowl and gently pull the bottom of the dough up and over the top. Give the bowl a quarter turn and repeat. Do this 2 more times until you have gone around the bowl in a full circle and pulled the dough 4 times. The dough will sit on top of itself and be taller than when you started folding. Cover the dough and set aside for another 45-60 minutes and then repeat turning and folding the dough. Repeat this process every 45-60 minutes until the dough has at least doubled in size, has lots of air bubbles up the side of the bowl, and feels lighter and supple when folding, 4-6 hours depending on the ambient temperature.
4. Gently turn the dough out onto a lightly floured work surface, taking care not to knock out the air. Do not flour the top of the sticky dough. Pull each side of the dough, in about 6 places, towards the center, letting it stick to itself, to make a rough circle. Using a bench scraper or large metal spatula, flip the dough over, seam side down, cover and let rest for 20 minutes before the final shaping.  
5. Line a medium bowl with a lint-free kitchen towel and generously dust with rice flour; set aside. Dust a work surface with bread flour. Using the bench scraper, flip the dough over and gently pull underneath into a loose rectangle with one short side closest to you. Fold one long side a little over halfway on the rectangle and then fold the other long side over top, similar to a letter fold, letting the dough stick to itself. Starting with the short end furthest from you, gently pull the dough away from you and then fold on top of the end of the rectangle. Continue pulling and folding until you have a squarish bundle. Flip the dough over. Use the bench scraper to tuck the ends under to the dough until you have a round. In one swoop, use the scraper to lift up the dough and put it seam side up in the prepared bowl. Cover and let rise at room temperature until slightly puffed, 45 minutes to 1 hour 30 minutes. A gentle press in the dough will keep a slight indent and not immediately spring back. Refrigerate overnight. 
6. Put a 5 QT HexClad Dutch Oven on the center rack of the oven and heat to 450ᵒ, for about 30 minutes. Put a small empty baking sheet on the rack below to absorb some heat and prevent the bottom of the bread from burning. Lightly scrunch a piece of parchment paper, to keep it from cutting into the dough. Remove the dough from the refrigerator, place the parchment against the top surface and carefully flip over. Evenly rub any excess flour from the top of the bread and cut a slash in the top using a sharp thin knife. Uncover the pot and, using the parchment, carefully lower the bread into the hot Dutch oven. Trim any excess parchment at the top, cover and bake for 20 minutes. Uncover and continue to bake until the bread is golden brown and hollow when tapped, 25 to 30 minutes more. 
7. Cool in the pot for 10 minutes and then use a spatula to remove to a rack. Cool until slightly warm or room temperature.